Optics for combining the light output from multiple light sources are common in the art, particularly for directional illumination, as illustrated in FIGS. 1A and 1B.
FIG. 1A illustrates the use of a compound parabolic reflector 110 that receives the light output from multiple light sources 101 via a light input surface 120 and emits the composite light from a light emitting surface 130. The light sources 101 may be mounted on a submount 105.
The side surfaces 115 of the parabolic reflector 110 may be coated with a reflective coating, or the light guide 110 may be encased in a second reflective material, or a material with an index of refraction that facilitates reflection by total internal reflection.
The light output patterns of conventional light sources 101 generally exhibit a Lambertian light output pattern, emitting substantially the same amount of light in each angular direction from the light emitting surface. Light that is emitted at near-orthogonal angles (not illustrated) from the light emitting surfaces of the light sources 101 may be emitted from the light exit surface 130 of the light guide directly, without reflection within the light guide 110. A substantial portion of the light emitted from the light emitting surfaces, on the other hand, will be emitted from the light exit surface 130 after one or more reflections at the side surfaces 115 of the light guide 110, as illustrated.
Because the surfaces 115 of the tapered light guide 110 are at an angle that is not orthogonal to the light emitting surfaces of the light sources 101, the angle at which the light is reflected from the surface, relative to the surfaces of the light sources 101, is closer to the orthogonal than the angle at which the light was emitted from the light sources 101. With each reflection from the sloped surfaces 115, the angle of reflection, relative to the light emitting surfaces, continues to become closer to the orthogonal. Accordingly, the light output from the exit surface 130 is more collimated than the light emitted from the surfaces of the light sources 101.
The curvature of the surfaces 115 is designed such that light at any angle coming from a point source at the center of the light guide 110 will be reflected in the orthogonal direction relative to the light emitting surface. That is, the tangent to the curve at each point on the surface 115 is at an angle relative to the surface of 90-A/2 degrees, where A is the angle from the point source to that point. If the light source were, in fact, a point source at the center of the light guide 110, all of the light reflected from the surfaces 115 would be reflected in the same direction, orthogonal to the surface, producing a highly directional light output from the light guide 110.
In an actual embodiment of a light source such as a semiconductor light emitting device, the light is emitted from the light emitting surface area of the light source, rather than an ideal single point in the center of the light guide 110. Light that is emitted from locations on the surfaces of the light sources 101 that are not at the center of the light guide will not strike the surface 115 at the proper point for being reflected orthogonal to the surface, resulting in a light output pattern that is less collimated than the light output from a single point in the center of the light guide 110.
Accordingly, a design goal for applications requiring a highly directional light output is to minimize the surface area of the light source to more closely resemble a point source. However, the amount of light that can be emitted by a semiconductor light emitting device is dependent upon the light emitting surface area of the device, and, in general, the greater the light emitting surface area, the greater the intensity (or brightness) of the light output. To achieve a light output that is very bright, and suitable for directional lighting, multiple light emitting devices are densely situated in the center of the light guide that serves to collimate the light output.
In some embodiments, the curvature of the surfaces 115 is modified so as not to ‘favor’ light that is emitted from the center of the light guide 110. That is, the curvature may be such that light emitted from the center of the light guide 110 is reflected at an angle that is not orthogonal, while light emitted from off-center locations are reflected at a more orthogonal angle. However, regardless of the modifications to the light guide, the light output from a light guide that receives light from a surface area will be less collimated than the light output from a light guide that receives light from a point source.
In most situations, particularly when the maximum deviation from the center of the light guide 110 is small, the lack of perfect collimation does not introduce adverse effects, other than producing a light output pattern having a wider beamwidth than the ideal. Consider, however, the effects when the surface area of the combined light sources 101 is large, and when the different color light sources 101 are not randomly distributed on the substrate 105.
FIG. 1B illustrates an example light guide 160 that is designed to accommodate a large number of light sources, 101R, 101G, and 101B, representing red, green, and blue light sources, respectively. For a variety of manufacturing reasons, arrays of multicolor light emitting devices are typically arranged in banks of each color on a substrate 105. In this example, a 9×9 array of light sources is arranged with a 3×9 bank of red light sources 101R, a 3×9 bank of green light sources 101G, and a 3×9 bank of blue light sources 101B.
Three light beams 180R, 180G, and 180B are illustrated as being emitted from a red light source 101R, a green light source 101G, and a blue light source 101B. Each of these beams 180R, 180G, 180B are emitted at the same angle relative to the surfaces of the light sources 101R, 101G, and 101B. In this example, the curvature of the side surface 165A of the light guide 160 is such that the light 180B is reflected at a near-orthogonal angle, relative to the exit surface 190. However, the light 180G, striking the surface 165 at a higher elevation, having a steeper slope, is emitted at an angle that farther from the orthogonal of exit surface 190 than the light 180B; and the light 180R, striking at an even higher elevation, is emitted at an angle that is even further from the same orthogonal.
On the side surface 165B, on the other hand, the opposite effect is produced. Light 185R is reflected at a near-orthogonal angle, while light 185B is reflected far off the orthogonal.
One of skill in the art will recognize that curvature of the sides 165A, 165B may be shaped differently, and a different optical effect will be produced. For example, the sides 165A, 165B may be shaped such that the light 180G, 185G from the center of the array is reflected at an orthogonal, or near orthogonal angle, rather than the light 180B striking side 165A and 185R striking side 165B, but this adjustment will only cause the light 180B and 185R to be reflected at an angle that is significantly off the orthogonal. In practice, the curvature (parabolic characteristics) are selected such that this non-uniform (different angles of reflection for different colors) is least noticeable, or least objectionable.
The overall effect of the non-uniform reflection pattern of FIG. 1B is that on the left side of the light emitting surface 190, very little light 185B from the blue light source 101B will exit the surface 190 at an orthogonal angle, while substantially more light 185R from the red light source 101R will exit the surface 190 at an orthogonal angle. On the right side of the light emitting surface 190, very little light 180R from the red light source 101R will exit at an orthogonal angle, while substantially more light 180B from the blue light source 101B will exit the surface 190 at an orthogonal angle. Light from the green light source 101G, on the other hand, will be symmetrically distributed across the left and right sides of the exit surface 190, albeit not orthogonal in this example.
This non-uniform distribution of the different colors and different emission angles of the colors presents a number of drawbacks, particularly in systems that are designed to provide a directional light output with a uniform mix of the colors, such as a system that produces a directional white light output. Viewed from an orthogonal direction, the right side of the surface 190 is likely to appear more blue than the left side, and the left side of the surface 190 is likely to appear more red than the right side. As the viewing angle changes, the intensity of the off-orthogonal light on one side will appear to increase while the intensity of the off-orthogonal light on the other side will appear to decrease.
It is significant to note that this non-uniform distribution of colors and emission angles is primarily caused by the optics 160 used to provide a directional, collimated light output. In an application that provides non-directional lighting, such as a retrofit light bulb having a wide field of illumination, the Lambertian light output from each of the light sources of different color naturally overlap each other, and will provide a similarly perceived output regardless of the angle of view.
In like manner, if each of the light sources were of the same color, or the different color sources were sufficiently randomly distributed on the substrate, or the different colors struck the input surface of the light guide 160 in a random manner, the different patterns of reflection on each side 165A, 165B of the light guide 160 would be immaterial, other than being a cause of an increased beamwidth compared to a truly collimated light output.
USP 2012/0069547 published for Gielen et al. on 22 Mar. 2012, provides a color-mixing optical element 250 situated between the light sources 101 and the parabolic reflector 210, as illustrated in FIG. 2. Light that is emitted from the light sources 101 at angles substantially off from orthogonal is reflected from the walls 255 of the color mixing element 250, increasing the likelihood that the light emitted from any particular light source 101 will exit the surface 220 and strike the walls 215 of the reflector 210 in a more randomly distributed pattern, thus producing a more uniform light output from the light emitting surface 230. Although light from the left and right sides of the surface 220 will be reflected differently from the walls 255 than light from the center of the surface 220, the particular color or pattern of the light from each side is more random, producing less of a color-specific non-uniformity on the surface 230.